A gluconic acid derivative, namely 2-keto-L-gulonic acid (hereinafter KLG), is a key intermediate in the production of valuable compounds including ascorbic acid (Vitamin C). However, to obtain high yields during the conversion of KLG to ascorbic acid, the KLG must be highly purified with a limited amount of impurities.
A convenient method for producing KLG is through a fermentation process. However, since most fermentation broths are maintained at neutral or near neutral pHs by the addition of basic substances, salts of KLG rather than KLG are produced. Furthermore, the fermentation broth also contains cells, neutrals and other undesirable materials. These additional components may interfere with the downstream chemistry used to convert KLG to ascorbic acid, therefore, the KLG must be isolated from the broth. Thus understood, any fermentation process for KLG must be integrated with an efficient recovery and purification process.
U.S. Pat. No. 5,747,306 discloses a method of separation using electrodialysis.
The viable fermentation broth is maintained at a near neutral or basic pH of between 5 and 9 by the addition of basic substances such as sodium hydroxide, potassium hydroxide or ammonia The broth is then passed through an electrodialysis tank which contains repeating cation and anion exchange membranes wherein the salts of KLG are removed from the broth. The viable fermentation broth containing neutrals, such as nutrients to insure survival of the microorganisms, is then recirculated into the fermentation system for reuse. However, the electrodialysis process produces a stream of purified KLG salt which gives lower yields during the conversion to ascorbic acid.
Additionally, the prior art process of converting organic sugars to the salt of KLG by using live and metabolically active microorganisms is complicated and demands constant vigilance to maintain a viable and active fermentation broth for the growth and/or metabolism of the microorganisms to ensure acceptable conversion of the substrate to the KLG salt.
To overcome the problems related with a high concentration of KLG salts during recovery of KLG from a fermentation medium, U.S. Pat. No. 4,990,441 discloses a method of acidifying the fermentation medium with sulfuric acid thereby precipitating the salt cation with the sulfate anion and protonating the KLG anion. However, it should be noted, that the medium containing salts of KLG also contains inorganic impurities such as phosphate and chloride anions which will also be converted to their corresponding acids with the addition of sulfuric acid. As a result, these inorganic acids can be concentrated during the KLG recovery processes, such as evaporative crystallization or direct drying and can cause acid catalyzed degradation of KLG. To rectify this problem the prior art contacts the medium containing the KLG and other impurities with a cation and anion exchange resin to remove any ionized impurities. But, neutral organics present in the fermentation broth which include simple and complex sugars may be unionized at the solution pH and thus not removed by passing the medium through cation and anion exchange resins. As such, the recovery of purified KLG is limited by the presence of these neutrals in the medium. The neutral sugars are concentrated during evaporative crystallization of KLG which causes increased viscosity of the mother liquor. As a result, multiple pass crystallization becomes difficult and KLG recovery is limited. Also, the neutrals interfere with direct drying which is a preferred method of recovery because of higher KLG yield with lower capital costs.
Accordingly, methods are needed for the concentration and purification of gluconic acid derivatives, such as KLG, which provide a higher recovery yield of KLG without the salts thereof, without contamination by inorganic impurities and neutral organics and/or without the need to maintain a fermentation medium for the growth and/or metabolism of a living and active culture of microorganisms.